The agency says it has 54 complaints from owners that engines stalled while the cars were being driven. In all cases the stalling happened after the cars were refueled. Forty-eight of the owners said they had several stalling incidents, and seven said they had trouble restarting the cars, according to documents posted on the NHTSA website.

Chrysler spokesman Eric Mayne said Tuesday that the company is cooperating in the investigation, and that it has no reports of any crashes or injuries. If customers suspect a problem, they should contact a Chrysler dealer, he said.

Investigators are checking the fuel vapor recovery system for possible failure, according to documents posted Tuesday on the agency’s website.

NHTSA said the number of complaints is increasing, with more than half coming in the past eight months.

In one complaint, a driver told NHTSA that on April 3, a 2006 Charger stalled while the car was in the center lane of a freeway during peak traffic hours. The driver put the Charger in neutral and tried to restart it, moving to the slow lane to get off the highway. “Car died two more times until I was off the freeway. I will not drive on the freeway with this car until it’s fixed,” the driver wrote.

Investigators will check how often the problem happens and determine if a recall is needed.

Mayne said the investigation is in its early stages and no defect has been identified.